gnupg: Agent HAVEKEY
2.6.11 Check whether a key is available
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This can be used to see whether a secret key is available. It does not
return any information on whether the key is somehow protected.
HAVEKEY KEYGRIPS
The agent answers either with OK or 'No_Secret_Key' (208). The
caller may want to check for other error codes as well. More than one
keygrip may be given. In this case the command returns success if at
least one of the keygrips corresponds to an available secret key.